Various techniques have been proposed as a technique for color tone control of a picture of a printing press.
For example, in techniques disclosed in Patent Document 1 and Patent Document 2, color tone control is performed in such a procedure as described below.
First, a spectral reflectance of a picture printed by printing units of different colors is measured by a spectrometer. Then, the spectral reflectance (average spectral reflectance in an overall key zone) is arithmetically operated for each of key zones of ink keys, and the spectral reflectance of each key zone is converted into a color coordinate value (L*a*b*) proposed by the International Commission on Illumination. If the ink supplying amount for each color is adjusted and test printing is performed and then a printing sheet (hereinafter referred to as OK sheet) having a desired color tone is obtained, then the color coordinate value for each key zone of the OK sheet is set as a target color coordinate value. Then, actual printing is started, and the difference (color difference) between the color coordinate values of the OK sheet and a printing sheet (printing sheet obtained by actual printing is hereinafter referred to as actual printing sheet) is calculated for each of the key zones. Thereafter, an increasing and decreasing amount for the opening of the ink key of each printing unit with respect to the color difference is calculated, and the opening of each ink key of each printing unit is adjusted by online control so that the color difference may be reduced to zero.
However, according to the techniques disclosed in Patent Documents 1 and 2, a spectrometer is used as a measurement section. The spectrometer requires a high cost. Further, where an object of measurement (in this instance, a printing sheet) moves at a very high speed as in the case of a rotary press for newspapers, the spectrometer cannot follow up the measurement object because of the processing capacity thereof. Further, in the method described above, since the color tone control is started after an OK sheet is printed, a great amount of paper loss appears after the printing process is started until the OK sheet is printed. Further, in the method described above, a picture in the key zone of each ink key is averaged over the entire key zone and the color tone control is performed based on the spectral reflectance after the averaging. Therefore, where the image area ratio of the picture in a key zone is low, a measurement error of the spectrometer increases and the control is likely to be rendered instable. Further, particularly severe color tone management is sometimes requested regarding a specific noticed point in a picture depending upon an order from a customer. Where the color tone control is to be performed for a specific noticed point in such a manner as just described, data such as CIP4 data [JDF (Job Definition Format) data of the CIP4 (International Cooperation for Integration of Prepress, Press, and Postpress) standard] must be received as image data to be used as a reference from an upstream plate making step and color separation must be performed for a control point so that the ink supplying amount is estimated.
Thus, Patent Document 3 discloses a technique wherein, in order to solve such subjects as described above, color tone control is performed in accordance with the following procedure.
First, a target color mixture halftone density for each ink supplying unit width when a printing picture is divided by the ink supplying unit width of an ink supplying apparatus is set. It is to be noted that, where the ink supplying apparatus is an ink key apparatus, the ink supplying unit width of the ink supplying apparatus is the key width (key zone) of each ink key, but where the ink supplying apparatus is a digital pump apparatus, the ink supplying unit width is the pump width of each digital pump. It is to be noted that a setting method for the target color mixture halftone density is hereinafter described.
If printing is started and an actual printing sheet is obtained, then an actual color mixture halftone density for each ink supplying unit width of the actual printing sheet is measured using an IRGB densitometer. Then, actual tone values for each ink color corresponding to the actual color mixture halftone density are determined based on a corresponding relationship set in advance between tone values and color mixture halftone densities for the individual ink colors. As a method for determining actual tone values from an actual color mixture halftone density, a database wherein a relationship between tone values and color mixture halftone densities for individual ink colors is stored, for example, a database wherein data obtained by printing a color scale of the Japan Color (ISO12642) for Newspaper Printing established by the ISO/TC130 National Commission and actually measuring the color scale by means of an IRGB densitometer are stored, may be used. More simply, the database can be utilized also to utilize an approximate value calculated using the known Neugebauer expression. Further, target tone values for each ink color corresponding to the target color mixture halftone density are determined based on the corresponding relationship described above between tone values and color mixture halftone densities. Different from the actual tone values, the target tone values need not be determined every time, but it is sufficient to determine the target tone values once unless the target color mixture halftone density varies. For example, the target tone values may be determined at a point of time when the target color mixture halftone density is set.
Then, an actual monochromatic halftone density corresponding to the actual tone values is determined based on a corresponding relationship set in advance between tone values and monochromatic halftone densities. As a method of determining an actual monochromatic halftone density from actual tone values, a map or a table which represents a relationship between monochromatic halftone densities and tone values may be prepared such that the actual tone values are applied to the map or the table. More simply, the relationship described above may be approximated using the known Yule-Nielsen expression to determine the actual monochromatic halftone density. Meanwhile, a target monochromatic halftone density corresponding to the target tone values is determined based on the corresponding relationship described above between tone values and monochromatic halftone densities. Different from the actual monochromatic halftone density, the target monochromatic halftone density need not be determined every time, and it is sufficient to determine the target monochromatic halftone density once unless the target tone values vary. For example, the target monochromatic halftone density may be determined at a point of time when the target tone values are set.
Then, a solid density difference corresponding to a difference between the target monochromatic halftone density and the actual monochromatic halftone density under the target tone values is determined based on a corresponding relationship set in advance among tone values, monochromatic halftone densities and solid densities. As a method of determining the solid density difference, a map or a table which represents the corresponding relationship described above is prepared, and then the target tone values, target monochromatic halftone density and actual monochromatic halftone density are applied to the map or table. More simply, the relationship described above may be approximated using the known Yule-Nielsen expression to determine the solid density difference. Then, the ink supplying amount is adjusted for each of the ink supplying unit widths based on the determined solid density difference and the ink supplying amount for each color is controlled for each of the ink supplying unit widths. The adjustment amount of the ink supplying amount based on the solid density difference can be determined simply using the known API (Auto Preset Inking) function.
According to such a picture color tone controlling method as described above, since color tone control can be performed using not a spectrometer but an IRGB densitometer, the cost required for the measuring system can be reduced, and besides the picture color tone controlling method can be applied sufficiently also to a high speed printing press such as a rotary press for newspapers.
Meanwhile, as a technique for setting a target color mixture halftone density where kcmy tone values data of a printing object picture (for example, image data for plate making or the like) can be acquired from the outside (for example, a printing requesting source or the like), the following technique has been proposed.
First, the acquired image data (kcmy tone values data) are used to set a noticed pixel (the noticed pixel may be a single pixel or a plurality of contiguous pixels in a mass) corresponding to each of ink colors for each ink supplying unit width from among pixels which form the printing object picture. Then, the tone values of the noticed pixel are converted into a color mixture halftone density based on a corresponding relationship set in advance between tone values and color mixture halftone densities. Then, the color mixture halftone density of the noticed pixel is set as a target color mixture halftone density, and the actual color mixture halftone density of the set noticed pixel is measured.
According to the proposed technique, since color development can be estimated in a unit of a pixel by utilizing the database of Japan Color (ISO12642) or the like, color tone control can be performed for a particular noticed point (noticed pixel) of the picture at a point of time immediately after printing is started without waiting that an OK sheet is printed. It is to be noted that the kcmy tone values data may be bitmap data of the printing object picture (for example, data for 1 bit-Tiff plating making). Or, low resolution data corresponding to CIP4 data obtained by conversion of such bitmap data may be used alternatively.
It is to be noted that, as a setting method of a noticed point (noticed picture), a method is available wherein an image of a printing picture is displayed on a display apparatus such as a touch panel using bitmap data such that an operator may designate a noticed point arbitrarily. Also a method has been proposed wherein a pixel having a maximum density sensitivity, or a pixel having a maximum autocorrelation to the tone values, is automatically extracted for each ink color through arithmetic operation and is set as a noticed pixel. In a particular setting method of a noticed pixel, an autocorrelation sensitivity H is introduced such that a pixel having a maximum autocorrelation sensitivity H is determined as a pixel having a maximum autocorrelation and is set as a noticed pixel. For example, the autocorrelation sensitivity Hc to cyan can be represented, using pixel area ratio data (c, m, y, k), as “Hc=cp/(c+m+y+k)” and a pixel having a maximum value of the autocorrelation sensitivity Hc is set as a noticed point of cyan. It is to be noted that p is an exponentiation of the autocorrelation.
If a pixel having a maximum autocorrelation with regard to tone values is extracted through arithmetic operation and set as a noticed pixel for each ink color and a target monochromatic halftone density and an actual monochromatic halftone density are calculated with regard to the noticed pixel and then the ink supplying amount is feedback controlled so that the actual monochromatic halftone density may approach the target monochromatic halftone density in such a manner as described above, then stabilized color tone control can be achieved.
Patent Document 1: Japanese Patent Laid-Open No. 2001-18364
Patent Document 2: Japanese Patent Laid-Open No. 2001-47605
Patent Document 3: Japanese Patent Laid-Open No. 2004-106523